The present invention relates to the field of auto-darkening eye protection devices, such as welding helmets having a shutter (or lens) assembly that automatically darkens upon the detection of a welding arc. A photosensitive device such as a photodiode or a phototransistor may be used to sense the intensity of light incident on the area of the shutter assembly so as to provide an indication to the circuitry controlling the shutter assembly that the shutter assembly needs to be driven to either a dark state or a clear state. If a welding arc is present, the welding helmet protects the eyes of the welder from any danger caused by the intensity of the welding arc by driving the shutter assembly to a dark state, thereby decreasing the amount of energy passing through the lens to the welder""s eyes. U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, 5,248,880, 5,252,817, 5,347,383, 5,533,206, 5,751,258, 5,959,705, 6,067,129, and 6,070,264 each disclose various shutter assemblies and liquid crystal driver electronics that can be used in conjunction with the present invention. The disclosures of these above-mentioned patents are hereby incorporated in their entireties by reference.
Commonly-owned U.S. Pat. No. 5,347,383 discloses a driving circuit for a liquid crystal shutter. The sensor circuitry of this invention utilizes a photodiode to detect the occurrence of welding. This sensor circuitry also utilizes a comparator to compare the sensed light signal with a threshold value to determine whether the shutter assembly should be driven to a dark or clear state. Additionally, the ""383 patent discloses the use of a 9 V supply.
While the invention disclosed in this patent functioned for its intended purpose, a need was felt for an improvement in the power consumption by the sensor circuit. As incident light increases on a photodiode, the voltage across the photodiode will begin to saturate. To prevent the photodiode from saturating, a steadily increasing load must be put on the photodiode which leads to excessive power consumption.
To alleviate the excessive power consumption inherent in a photodiode-based sensor circuit, a phototransistor has been utilized as a weld sensor. The use of a phototransistor allows the use of feedback to bias the phototransistor so that less current is needed to keep the phototransistor in its operational mode. Commonly-owned U.S. Pat. Nos. 5,252,817, 5,248,880, 5,751,258, and 6,070,264 are illustrative of sensor circuits using phototransistors as weld sensors. Each of these patents discloses a sensor circuit wherein the output of the phototransistor is fed into a comparator. The comparator compares the phototransistor output with a threshold level. If the phototransistor output exceeds the threshold level, the drive circuitry is activated to darken the shutter assembly. If the phototransistor output does not exceed the threshold level, the drive circuitry operates the shutter assembly in a clear state. While the circuits disclosed in these patents utilize feedback to bias the phototransistor and avoid the excessive drawing of current, heavy loads were still needed. The circuits implementing such designs used voltage supplies ranging from 5.6 V to 9 V. Therefore, a need was still felt for a sensor circuit having improved power consumption characteristics.
Moreover, the phototransistors used in the prior art designs were metal can phototransistors. Metal can phototransistors are relatively big and bulky. Their size, height and relative difficulty in mounting serves as a limiting factor in the ability of designers to reduce the size of the units in which the sensor circuit is implemented. Thus, a need was felt to use a smaller and more compact phototransistor that is more easily mountable to a circuit board to produce a smaller, sleeker unit while still having the ability to maintain a constant signal level without excessive loading or the drawing of excessive current.
Additionally, the sensor circuits of the prior art produced an output voltage from the phototransistor in response to incident light intensity as seen in FIG. 3 of the ""880 and ""817 patents. As can be seen, low light intensities produce a steep rise in output of the phototransistor. Because of the power drain caused by the response of the phototransistor to low intensity incident light, it is desirable that the phototransistor be configured to minimize the phototransistor output signal when the sensor circuit is exposed to low intensity incident light. Thus, a need existed for a sensor circuit that provided greater attenuation in the response of the phototransistor to low intensity incident light.
While it is desirable to minimize the phototransistor output when the sensor circuit is exposed to low intensity incident light, the phototransistor still must be able to quickly increase its output in response to a transition from low intensity light to high intensity light, such as the light provided by a welding arc. Thus, an ever present need exists within the art to sharpen the rise provided by the phototransistor in response to sharp increases in light intensity.
Also, the sensor circuitry of the prior art used a comparator to correlate the sensed light signal with the desired shade level. The comparator compared the output of the phototransistor with a threshold voltage signal to determine whether the shutter assembly should be driven to a dark state or a clear state. This design required additional circuitry to set the threshold voltage level. This additional circuitry not only complicated circuit design, but also increased the drain on the power supply. Thus, a need was felt to simplify the sensor circuitry to provide a more power-efficient way of correlating the phototransistor output to the proper shade setting of the shutter assembly.
In order to solve these and other problems in the prior art, the inventor herein has succeeded in designing and developing an improved welding detection circuit utilizing a novel phototransistor-based sensor circuit. This sensor circuit comprises a phototransistor biased via a feedback circuit and having an output connected to an amplifier. The sensor circuit can be connected to a power supply and a control circuit to drive a shutter assembly to either a dark state or a clear state depending upon the intensity of incident light.
One feature of the present invention is the use of a resistor coupled between the base and emitter of the phototransistor. This resistor helps reduce the current produced by the sensor during low ambient light conditions, thereby attenuating the phototransistor output in response to low intensity light signals, and helps produce a sharply rising voltage from the phototransistor in response to high intensity light signals. Preferably, the feedback circuit also includes a second resistor coupled between the emitter of a feedback transistor and ground to further attenuate phototransistor output in the presence of low intensity ambient light.
Another feature of the present invention is its use of a planar phototransistor. Because of the planar phototransistor""s small size, as compared to the metal can phototransistors used in the prior art, and because of the planar transistor""s ability to maintain a constant signal level without excessive loading or the drawing of excessive current, the use of a planar phototransistor not only performs as well as metal can phototransistors, but also allows a reduction in the size of the unit in which the circuit is implemented. Preferably, the planar phototransistor is configured for a surface mount to further simplify construction of the circuit.
Another feature of the present invention is its use of a closed loop noninverting amplifier to provide a gain for the phototransistor output. The gain of the amplifier is preferably set so that a sufficient output voltage will be generated to activate the shutter assembly when the phototransistor produces an output indicative of the presence of a welding arc. Preferably, a capacitor is coupled between the phototransistor output and noninverting input of the amplifier to block the DC portion of the phototransistor output.
Another feature of the present invention is its use of the energy saved by an improved and efficient circuit design to recharge a rechargeable battery. By recharging the battery, the present invention extends the battery life of the invention""s power supply.
Another feature of the present invention is its use of a solar cell to reduce the circuit""s power consumption. By using a solar cell to power various components of the circuit, the present invention prevents those components from acting as a drain on the power supply when the invention is left unexposed to light. Often, while not in use, a welding helmet will be left in a dark room or left face down on a table. When in these conditions, it is undesirable for the circuit to operate as a drain on the power supply. When the welding helmet is in use, it will be either outdoors, in a lighted room, or in a dark environment with the presence of welding arc. In such conditions, it is desirable to use the light incident on the welding helmet to power the circuitry therewithin.
The present invention uses the solar cell to power the phototransistor and the amplifier that is coupled to the output of the phototransistor, thus preventing those two components from draining the power supply when the welding helmet is left unexposed to light.
The present invention also uses the solar cell to power an activation circuit, the activation circuit functioning to activate a signal generator. The signal generator, once activated, generates the voltage level and frequency signal to be used to drive the shutter assembly to a dark state. The generation of this signal acts as a drain on the power supply. By using the solar cell to power the activation circuit, the present invention improves the circuit""s power consumption by triggering the signal generator when light is incident on the welding helmet.
Yet another feature of the present invention is its use of a selector circuit for selecting the drive signal that will be delivered to the shutter assembly. If the sensor circuit indicates to the selector circuit that a welding arc is present, the selector circuit will cause a dark state drive signal to be delivered to the shutter assembly. If the sensor circuit indicates to the selector circuit that no welding arc is present, the selector circuit will cause a xe2x80x9cclear statexe2x80x9d drive signal to be delivered to the shutter assembly. The selector circuit uses a transistor as a switch to control the selection of the drive signal. An RC circuit is part of the selector circuit. The RC circuit utilizes its RC time constant to delay the transition of the xe2x80x9cdark statexe2x80x9d drive signal to the xe2x80x9cclear statexe2x80x9d drive signal, thus preventing the shutter assembly from switching to a clear state during brief xe2x80x9coffxe2x80x9d periods in the weld pulsations that exist with various weld types.
While the principal advantages and features employed are explained above, a fuller understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follows.